Introduction

Gas chromatography (GC) separates a mixture of chemicals (a sample)
into individual components. These components (or analytes) can then be
analyzed for their identity and their concentration in the sample. In
general, analytes must be volatile or semivolatile (able to be vaporized)
in order to be anlyzed on the GC. Analytes must also be dissolved in an
organic solvent at very low concentrations.

The uses for GC are numerous. They are used extensively in the medical,
pharmacological, environmental, and law enforcement fields. SRIF has two
GC systems. One has an electron capture detector (GC-ECD) and the other
has a mass spectrometer (GC-MS). They very popular instrument and are used
for many types of academic projects and research. This paper discusses the
GC-ECD and another SRIF background paper discusses the GC-MS.

General Chromatography

In all chromatography, separation occurs when the sample mixture is
introduced (injected) into a mobile phase. In liquid chromatography
(LC), the mobile phase is a solvent. In gas chromatography (GC), the
mobile phase is an inert gas such as helium.

The mobile phase carries the sample mixture through what is referred
to as a stationary phase. The stationary phase is a usually
chemical that can selectively attract components in a sample mixture. The
stationary phase is usually contained in a tube of some sort. This tube is
referred to as a column. Columns can be glass or stainless steel of
various dimensions.

The mixture of compounds in the mobile phase interacts with the
stationary phase. Each compound in the mixture interacts at a different
rate. Those that interact the fastest will exit (elute from) the column
first. Those that interact slowest will exit the column last. By changing
characteristics of the mobile phase and the stationary phase, different
mixtures of chemicals can be separated. Further refinements to this
separation process can be made by changing the temperature of the
stationary phase or the pressure of the mobile phase.

Gas Chromatography

SRIF's GC has a long, thin column containing a thin interior coating of
a solid stationary phase (5% phenyl-, 95% dimethylsiloxane polymer). This
0.25 mm diameter column is referred to as a capillary column. This
particular column is used for semivolatile, non-polar organic compounds
such as the PAHs we will look at. The compounds must me in an organic
solvent.

The capillary column is held in an oven that can be programmed to
increase the temperature gradually (or in GC terms, ramped). this helps
our separation. As the temperature increases, those compounds that have
low boiling points elute from the column sooner than those that have
higher boiling points. Therefore, there are actually two distinct
separating forces, temperature and stationary phase interactions mentioned
previously.

As the compounds are separated, they elute from the column and enter a
detector. The detector is capable of creating an electronic signal
whenever the presence of a compound is detected. The greater the
concentration in the sample, the bigger the signal. The signal is then
processed by a computer. The time from when the injection is made (time
zero) to when elution occurs is referred to as the retention time
(RT).

While the instrument runs, the computer generates a graph from the
signal. (See figure 1). This graph is called a chromatogram. Each
of the peaks in the chromatogram represents the signal created when
a compound elutes from the GC column into the detector. The x-axis shows
the RT, and the y-axis shows the intensity (abundence) of the signal. In
Figure 1, there are several peaks labeled with their RTs. Each peak
represents an individual compound that was separated from a sample
mixture. The peak at 4.97 minutes is from dodecane, the peak at 6.36
minutes is from biphenyl, the peak at 7.64 minutes is from chlorobiphenyl,
and the peak at 9.41 minutes is from hexadecanoic acid methyl ester.

Figure 1: Chromatogram generated by a GC.

If the GC conditions (oven temperature ramp, column type, etc.) are the
same, a given compound will always exit (elute) from the column at nearly
the same RT. By knowing the RT for a given compound, we can make some
assumptions about the identity of the compound. However, compounds that
have similar properties often have the same retention times. Therefore,
more information is usually required before an analytical chemist can
make an identification of a compound in a sample containing unknown
components.